[media] dvb_usb_v2: replace Kernel userspace lock with wait queue

There was sync mutex which was held over userspace. That is very
wrong and could cause deadlock if different userspace process is
used to "unlock". Wait queue seems to be correct solution for
that kind of synchronizing issue so use it instead.
lock debug gives following bug report:
================================================
[ BUG: lock held when returning to user space! ]
3.9.0-rc1+ #38 Tainted: G           O
------------------------------------------------
tzap/4614 is leaving the kernel with locks still held!
1 lock held by tzap/4614:

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>